This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-048166, filed Feb. 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a method of producing water-soluble chain-extended gelatin and, more particularly, to gelatin of this type exhibiting improved filtering characteristics when redissolved. The present invention further relates to the use of gelatin thus produced as a photographic component.
Silver salt light-sensitive materials have advanced by using a silver halide and gelatin. In particular, gelatin has a wide variety of effects on photographs and participates in all steps from the beginning to the end of silver salt photography, e.g., preparation of silver halide grains, coating, drying, storage, photographing, processing, and image storage. As the recent silver salt light-sensitive material preparation technologies are becoming precise, requirements for photographic gelatin are also becoming extremely strict.
Efforts have been made to meet these requirements, and one method is to modify gelatin. Of various modifying methods, U.S. Pat. No. 5,187,259 or Jpn. Pat. Appln. KOKAI Publication No. 56-2324 has disclosed a method of extending the chain (increasing the molecular weight) of gelatin while the gelatin is kept water-soluble (so-called water-soluble chain-extended gelatin). This water-soluble chain-extended gelatin is useful, on a support layer in the production of photographic elements, as a disperse dye carrier layer in the production of microfilm products or as a rapid film hardener carrier layer in color emulsion products. U.S. Pat. No. 5,187,259 and Jpn. Pat. Appln. KOKAI Publication No. 56-2324 describe that the viscosity of a solution using this gelatin is high, so a high coating rate can be used when the gelatin is applied to curtain coating. Additionally, the protective colloid properties of water-insoluble photographically useful substances, such as silver halide grains and disperse dyes, can be improved on the basis of the increased molecular weight.
Since the molecular weight is increased, however, the viscosity of the gelatin solution rises to increase the loads on individual operations. Also, even a low-concentration solution has a high gelation rate, so the solution is difficult to handle during the production. Furthermore, the effect on the yield is large, and this influences the producing cost.
When this gelatin is used, therefore, purification by an ion-exchange resin or ultrafiltration is impractical for the above reasons. Hence, it is necessary to perform drying and powdering by minimizing the number of producing steps. When this is the case, however, producing problems (coating suitability and coated surface condition failure) caused by the filtering characteristics arise if the gelatin is used as a photographic component. Any patents including the aforementioned disclosed patents do not describe improves of these problems after the reaction. So, it has been desired to improve the filtering characteristics of a solution when powder gelatin is redissolved.
It is an object of the present invention to provide a water-soluble chain-extended gelatin producing method which improves the filtering characteristics of a solution prepared by redissolving a powder of gelatin, without applying any producing load.
The above object is achieved by the following means.
(1) A method of producing a water-soluble chain-extended gelatin, comprising the steps of:
subjecting unreacted gelatin (to be referred to as source gelatin hereinafter) a source gelatin having a first isoelectric point, in aqueous solution, to a partial crosslinking reaction to form a reaction mixture comprising a partially crosslinked gelatin having a second isoelectric point;
filtering the reaction mixture;
adjusting a pH value of the filtered reaction mixture to a value equivalent to the second isoelectric point xc2x11.5; and
concentrating, drying and pulverizing the pH-adjusted reaction mixture.
(2) The method described in (1), wherein the partial crosslinking reaction is performed by using a compound selected from the group consisting of a bis-(vinylsulfonyl) compound and a compound capable of activating a carboxyl group.
(3) The method described in (1), wherein the partial crosslinking reaction is performed by reacting the source gelatin with a compound selected from the group consisting of a bis-(vinilsulfonyl) compound and a compound capable of activating a carboxyl group in an amount of 0.25 to 10 mmol per 100 g of source gelatin, at a temperature of 40 to 65xc2x0 C., and a pH of not less than a value equivalent to the first isoelectric point for 1 to 8 hours wherein the source gelatin is in a form of an aqueous solution with a gelatin concentration of 6 to 25% by mass.
(4) The method described in (2) or (3), wherein the bis-(vinylsulfonyl) compound is selected from compounds represented by formulas 1 and 2: 
where each R1 independently represents a hydrogen atom, an alkyl group, an aralkyl group or an aryl group, wherein these groups may be substituted, and n represents 0 or 1; 
where Y represents a vinyl group, A represents a single bond or a divalent coupling group, and each R2 independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.
(5) The method described in claim (2) or (3), wherein the compound capable of activating a carboxyl group is selected from compounds represented by formulas 3 and 4: 
where each of R3 and R4 independently represents an alkyl group, an aralkyl group, or an aryl group, wherein R3 and R4 may be combined to form a heterocycle together with the nitrogen atom; R5 represents an alkyl group, an alkoxy group, a dialkylamino group, or an N-alkylcarbamoyl group, and Xxe2x88x92 represents a monovalent anion, wherein if R5 contains, as a substituent, a sulfo group, a sulfoxy group, or a sulfoamino group, the compound may form an intramolecular salt without Xxe2x88x92; 
where R6 represents an alkyl group, a cycloalkyl group, an alkoxyalkyl group, or an aralkyl group, and R7 represents the group defined by R6 of a group represented by formula 5: 
where R8 represents an alkylene group, each of R9 to R11 independently represents an alkyl group, wherein one of R9 to R11 may be a hydrogen atom, and two of R9 to R11 may be combined to form a heterocycle together with the nitrogen atom.
(6) The method described in (3), wherein the source gelatin is selected from the group consisting of an acid-processed bone gelatin and a lime-processed bone gelatin.
(7) The Water-soluble chain-extended gelatin produced by the method described in any one of (1) to (6).
(8) A silver halide photographic light-sensitive material comprising the water-soluble chain-extended gelatin produced by the method described in any one of (1) to (6).
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
Concerning a partial crosslinking reaction in a method of producing water-soluble chain-extended gelatin of the present invention, U.S. Pat. No. 5,187,259 and Jpn. Pat. Appln. KOKAI Publication No. 56-2324 may be refered. In this partial crosslinking reaction, a bis-(vinylsulfonyl) compound or a compound capable of activating a carboxyl group is used.
First, a bis-(vinylsulfonyl) compound will be explained below.
This bis-(vinylsulfonyl) compound is preferably selected from compounds represented by formulas 1 and 2 below. 
In formula 1, R1 represents a hydrogen atom, an alkyl group having 1 to 20 carbon atoms (e.g., a methyl group, ethyl group, iso-propyl group, or n-butyl group), an aralkyl group having 6 to 20 carbon atoms (e.g., a benzyl group or phenethyl group), or an aryl group having 5 to 20 carbon atoms (e.g., a phenyl group, naphthyl group, or pyridyl group). These groups may be substituted. Examples of the substituent are a sulfonic acid group, a hydroxyl group, and a carboxyl group. R1 is particularly preferably a hydrogen atom. n represents 0 or 1, preferably 0.
Practical examples of a compound represented by formula 1 will be presented below. However, the present invention is not limited to these examples.
xe2x80x83CH2xe2x95x90CHSO2CH2SO2CHxe2x95x90CH2xe2x80x83xe2x80x83H-1xe2x80x83CH2xe2x95x90CHSO2CH2OCH2SO2CHxe2x95x90CH2xe2x80x83xe2x80x83H-3
Formula 2 will be described below. 
In formula 2, Y represents a vinyl group. A represents a single bond or a divalent coupling group. Although this divalent coupling group may be any group, it is preferably a cyclic or acyclic alkylene group having 1 to 10 carbon atoms in which one to three carbon atoms may be replaced by hetero atoms such as N, S, or O. The divalent coupling group is more preferably a chainlike hydrocarbon group having 1 to 5 carbon atoms. If the number of carbon atoms is 2 to 6, the group may be branched or straight-chained. This chain may also have a substituent such as an alkoxy (e.g., methoxy or ethoxy), a halogen (e.g., chloro or bromo), a hydroxy or an acetoxy. Each R2 independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. Practical examples of a compound represented by formula 2 will be presented below, but the present invention is not restricted to these examples.
CH2xe2x95x90CHSO2CH2CONHCH2CH2NHCOCH2SO2CHxe2x95x90CH2xe2x80x83xe2x80x83H-6
CH2xe2x95x90CHSO2CH2CONHCH2CH2CH2NHCOCH2SO2CHxe2x95x90CH2xe2x80x83xe2x80x83H-7
CH2xe2x95x90CHSO2CH2CONHCH2NHCOCH2SO2CHxe2x95x90CH2xe2x80x83xe2x80x83H-8
CH2xe2x95x90CHSO2CH2CONHxe2x80x94NHCOCH2SO2CHxe2x95x90CH2xe2x80x83xe2x80x83H-9 

A compound capable of activating a carboxyl group in gelatin will be described below.
This compound capable of activating a carboxyl group in gelatin is preferably selected from groups represented by formula 3 below and formula 4 to be presented later. 
In formula 3, each of R3 and R4 independently represents an alkyl group having 1 to 10 carbon atoms (e.g., a methyl group, ethyl group, or 2-ethylhexyl group), an aralkyl group having 7 to 10 carbon atoms (e.g., a benzyl group or phenethyl group), or an aryl group having 6 to 15 carbon atoms (e.g., a phenyl group or naphthyl group). R3 and R4 preferably combine to form a heterocycle together with the nitrogen atom. Examples of the ring are a pyrrolidine ring, a piperazine ring, and a morpholine ring, and a morpholine ring is particularly preferred. R5 represents a hydrogen atom, a halogen atom, a carbamoyl group, a sulfo group, an ureido group, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or a dialkylamino group having 2 to 20 carbon atoms. If R5 is an alkoxy group, an alkyl group, a dialkylamino group, or an N-alkylcarbamoyl group, these groups may be further substituted. Examples of the substituent are a halogen atom, a carbamoyl group, a sulfo group, a sulfoxy group, and an ureido group. Xxe2x88x92 represents a monovalent anion which is a counter ion of N-carbamoylpyridinium salt. If a substituent of R5 contains a sulfo group, a sulfoxy group, or a sulfoamino group, the compound may form an intramolecular salt without Xxe2x88x92. Preferred examples of the monovalent anion are halide ion, sulfuric acid ion, sulfonate ion, ClO4xe2x88x92, BF4xe2x88x92, and PF6xe2x88x92. It is particularly preferable to form intramolecular salt.
Practical examples of a compound represented by formula 3 will be presented below. However, the present invention is not limited to these examples. 
Formula 4 will be explained below.
R6xe2x80x94Nxe2x95x90Cxe2x95x90Nxe2x80x94R7
wherein R6 represents an alkyl group having 1 to 10 carbon atoms (e.g., a methyl group, ethyl group, or 2-ethylhexyl group), a cycloalkyl group having 5 to 8 carbon atoms (e.g., a cyclohexyl group), an alkoxyalkyl group having 3 to 10 carbon atoms (e.g., a methoxyethyl group), or an aralkyl group having 7 to 15 carbon atoms (e.g., a benzyl group or phenethyl group).
R7 represents any of the groups defined by R6 or a group represented by formula 5. 
wherein R8 represents an alkylene group having 2 to 4 carbon atoms (e.g., an ethylene group or propylene group). Each of R9 to R11 independently represents an alkyl group having 1 to 6 carbon atoms (e.g., a methyl group or ethyl group). One of R9 to R11may be a hydrogen atom. Also, two of R9 to R11 preferably combine to form a heterocycle (e.g., a pyrrolidine ring, piperazine ring, or morpholine ring) together with the nitrogen atom. R9 to R11 may also be substituted, and preferred examples of the substituent are a substituted or nonsubstituted carbamoyl group and sulfo group. Xxe2x88x92 represents a monovalent anion, and examples are halide ion, sulfuric acid ion, sulfonate ion, ClO4xe2x88x92, BF4xe2x88x92, and PF6xe2x88x92. If R10 is substituted by a sulfo group, the group represented by formula 5 may form an intramolecular salt without Xxe2x88x92.
Practical examples of a compound represented by formula 4 will be presented below. However, the present invention is not limited to these examples. 
Favorable partial crosslinking reaction conditions in the present invention are: the addition amount of a bis-(vinylsulfonyl) compound represented by formula 1 or 2 above or of a compound capable of activating a carboxyl group, represented by formula 3 or 4 above, is 0.25 to 10 mmol per 100 g of the dried mass of the source gelatin to be chain-extended; the reaction temperature is 40 to 65xc2x0 C.; the reaction pH value is equivalent to the isoelectric point of the source gelatin or more; the reaction time is 1 to 8 hr; and the reaction gelatin concentration is 6 to 25 mass %. More favorable partial crosslinking reaction conditions are: the addition amount of a bis-(vinylsulfonyl) compound represented by formula 1 or 2 or of a compound capable of activating a carboxyl group, represented by formula 3 or 4, is 0.25 to 8 mmol per 100 g of the dried mass of the source gelatin to be chain-extended; the reaction temperature is 45 to 60xc2x0 C.; the reaction pH value ranges from a value equivalent to the isoelectric point of the source gelatin to a value equivalent to the isoelectric point +3; the reaction time is 1 to 6 hr; and the reaction gelatin concentration is 6 to 20 mass %. Most favorable partial crosslinking reaction conditions are: the addition amount of a bis-(vinylsulfonyl) compound represented by formula 1 or 2 or of a compound capable of activating a carboxyl group, represented by formula 3 or 4, is 0.25 to 5 mmol per 100 g of the dried mass of the source gelatin to be chain-extended; the reaction temperature is 50 to 60xc2x0 C.; the reaction pH value ranges from a value equivalent to the isoelectric point of the source gelatin to a value equivalent to the isoelectric point +2.5; the reaction time is 1 to 5 hr; and the reaction gelatin concentration is 7 to 18 mass %. A bis-(vinylsulfonyl) compound represented by formula 1 or 2 or a compound capable of activating a carboxyl group, represented by formula 3 or 4, can be added at once in the form of an aqueous or alcohol solution or can be added by dropping the solution over 30 min to 3 hr. This compound is preferably added by dropping its solution over 30 min to 2 hr. Particularly preferably, the compound is added by dropping its solution over 30 min to 1.5 hr. The concentration of the solution is preferably 0.5 to 5 mass %, and more preferably, 0.5 to 2 mass %.
Gelatin to be chain-extended will be explained below. The major supply sources of photographic gelatin are beef skins and bones. Although the both materials can be used, gelatin produced from bones is preferred. Also, the source gelatin is roughly classified in accordance with the processing method. Although both of acid-processed gelatin and alkali(lime)-processed gelatin can be used, alkali(lime)-processed gelatin is preferred. The isoelectric points of acid-processed gelatin and alkali(lime)-processed gelatin are different. On the other hand, in the crosslinking reaction mentioned earlier there is almost no difference between the isoelectric point of the source gelatin to be extended and the isoelectric point of the chain-extended gelatin. Therefore, two or more of different source gelatins to be extended can be used. When this is the case, however, the difference between the isoelectric points of these source gelatins is preferably 1.5 or less, and more preferably, 1 or less.
When the above partial crosslinking reaction completes, the steps of filtration, concentration, drying, and powdering start. The present invention is characterized in that a pH adjusting step is inserted between filtration and concentration. That is, after a reaction aqueous solution obtained by the partial crosslinking reaction described above is filtered, the pH value of this reaction aqueous solution is adjusted within the range of value equivalent to the isoelectric point of the produced water-soluble chain-extended gelatin xc2x11.5. This pH value preferably ranges from the value equivalent to the isoelectric point xe2x88x920.5 to the value equivalent to the isoelectric point +1.0, and more preferably, from the value equivalent to the isoelectric point xe2x88x920.5 to the value equivalent to the isoelectric point +0.5. This makes it possible to improve the filtering characteristics of a solution when powder gelatin is redissolved.
Preferred examples of an acid used in the adjustment are sulfuric acid, hydrochloric acid, and nitric acid. Preferred examples of an alkali used in the adjustment are NaOH and KOH. Of these examples, sulfuric acid and NaOH are most preferred as an acid and alkali, respectively. The partial crosslinking reaction is preferably performed at pH equal to or higher than the isoelectric point of the source gelatin. So, it is desirable to adjust the pH within the target range by a minimum necessary amount of an acid. The concentration of an acid or alkali used in the adjustment is preferably 1 to 5 mol/L, and more preferably, 1 to 3 mol/L. The temperature of the pH adjustment is preferably 40 to 60xc2x0 C., and more preferably, 40 to 50xc2x0 C.
Subsequently, gelatin having a proper grain size is obtained through the steps of concentration, drying, and powdering. The series of steps can be performed by using methods described in known patents and scientific literature. The water-soluble chain-extended gelatin thus obtained can be used in a photographic element. This photographic element is appropriately a material sensitive to light, laser, or X-ray irradiation. The element is selected from a black-and-white reversal film, black-and-white film, color negative film, color reversal film, film on which a light-sensitive photographic component is digital-scanned, black-and-white reversal paper, black-and-white paper, color paper, reversal color paper, and paper on which a light-sensitive photographic component is exposed to laser irradiation from a digital database. The photographic element is particularly preferably a color negative film. One example is Jpn. Pat. Appln. KOKAI Publication No. 11-305396, the disclosure of which is incorporated herein by reference.
The gelatin was redissolved and added to various components, and these photographic elements were coated with the resultant gelatin solution. Consequently, good filtering characteristics of the gelatin solution caused no surface condition failure and imparted producing suitability.